Hydraulic steering systems are commonly used in agricultural vehicles, such as tractors and harvesters, that require power-assisted steering. The steering motor comprises a pump that is rotated by the torque applied to the steering wheel and a distribution valve. The distribution valve acts to direct the high pressure hydraulic fluid, herein termed oil for simplicity, generated by the internal pump to hydraulic steering actuators connected to the steerable wheels and to direct oil returning from the actuators to reservoir. The construction of the steering actuators is not material to the present invention and they may comprise two separate hydraulic jacks or a cylinder with a double acting piston.
The term “hydraulic steering motor” is used in its normally accepted sense to refer to a device that incorporates an internal pump, a distribution valve, an input shaft which in use is coupled for rotation by the steering shaft of a vehicle steering wheel, and connectors for hydraulic lines leading to a high pressure source of oil, to an oil reservoir and to hydraulic actuators connected to the steerable wheels of the vehicle.
In this way, as the steering wheel is turned, oil is supplied to the steering actuators to move the steerable wheels in the required direction and by an amount determined by the extent of rotation of the steering wheel.
In order to reduce the driver effort, an auxiliary pump is connected to the steering motor to supply it with oil under high pressure. The distribution valve serves to apply the high pressure oil to the steering actuators in a manner to supplement the oil pressure generated by the internal pump, thereby providing power assistance.
Thus, in the absence of an external high pressure source, the torque applied by the driver to drive the internal pump of the steering motor is of itself sufficient to steer the vehicle. When high pressure oil is available from the auxiliary pump, however, it is applied through the distribution valve to the wheel actuators in parallel with the oil from the internal pump so that the wheels are turned by the same desired amount but without as much driver effort.
Conventionally, the input shaft of the steering pump is directly coupled to the lower end of the steering shaft on which the vehicle steering wheel is mounted and the body of the steering pump is located within the driver's cabin.
A problem of such conventional mounting of the steering pump is that space within the cabin is often at a premium and the steering pump can prove an unpleasant source of noise.